Track circuit



R. C. BUCK TRACK CIRCUIT Aug. 16, 1966 2 Sheets-Sheet 1 Filed Sept.

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INVENToR. R. C. BU C K HIS ATTORNEY R. C. BUCK TRACK CIRCUIT Aug. 16, 1966 Filed Sept. 7, 1962 2 Sheets-Sheet 2 HIS ATTORNEY United States Patent O 3,267,286 TRACK ClRCUKT Roger C. Buck, Rochester, NX., assigner to General Signal Corporation, a corporation of New York Filed Sept. 7, 1962, Ser. No. 221,983 17 Claims. (Cl. 246-34) This invention relates to track circuits and more particularly to a fail-safe system for indicating the condition of a track section.

Various types of track circuits are employed for indicating presence of a train on a section of track. Generally, such circuits also provide -a warning of broken rails or other defective conditions along the track. These circuits usually include at least one relay coupled to one end of the track section, and a source of energy coupled to the rails at the other end of the track section. The relay is normally energized when the track section is unoccupied and not electrically defective. A train present on the track section completes a circuit between the rails, thereby shunting the track source and the relay, causing the relay to drop. A broken rail opens the circuit, also causing the relay to drop.

Certain problems peculiar to track circuits complicate the problem of designing a reliable track circuit. For example, rail and ballast resistance may vary considerably, depending upon weather conditions. It is obviously desirable to design a track circuit which is insensitive to such resistance changes while yet being sensitive to resistance changes between rails due to shunting caused by presence of a train `on the track. Another problem peculiar to track circuits is that of galvanic action between the rails and ballast. This causes flow of current which may aid or oppose current produced by the track battery. Again, it is highly desirable to design a track circuit which is insensitive to flow of current created th-rough galvanic action while yet being sensitive to cur- Irent changes due to shunting of the rails by presence of a train, as well as still being sensitive to current changes due to open or short circuits.

Furthermore, in design of track circuits utilizing relays, it is practically impossible to design the circuits so that the track relay will pick up and drop away at nearly identical values of relay current. However, such condition would be highly desirable for reliable track circuit operation, since the track relay would be released at higher values of train shunt.

Active circuit elements such as tubes and transistors have not generally been incorporated in track circuit for Various reasons; for example, tubes present shock and burn-out problems, while both tubes and transistors require circuits incorporating large numbers of cornponents, increasing possibility of failure due to transient voltage and current destruction. Furthermore, the impedance level at which transistors operate makes them highly prone to transient noise pick-up and interference.

Magnetic amplifier circuits containing passive circuit elements and diodes are generally undesirable for use in track circuits since the large number of diodes and passive components required in magnetic amplifier circuits increase possibilities of circuit failure.

To avoid the aforementioned track circuit problems, the present invention incorporates multi-aperture ferrite cores having substantially square hysteresis loops, in circuits responsive to track conditions. Multi-aperture ferrite cores enable design of extremely low impedance circuits which are not susceptible t-o transient noise pick-up. In addition, few components are necessary in such ferrite core circuits, thereby greatly reducing chances of failure. Circuits utilizing multi-aperture core devices can be made fail-safe, and are insensitive to temperature changes over a wide range of temperatures. Furthermore, normal variation of prime current of any core due to ballast and rail resistance changes as well as galvanic action between the ballast and .rails leaves the output of the multi-aperture device relatively unaffected. Therefore, one object of this invention is to provide a rugged, reliable track circuit which is relatively unaft'ected by normal ballast and rail resistance variations as well as by galvanic `action between the ballast and rails.

Another object is to provide a highly sensitive failsafe track circuit dependent for -its operation upon multiaperture ferrite cores.

Another object is to provide a track circuit utilizing multi-aperture ferrite cores which can be used as a building-block in any system requiring an indication of track condition.

Another lobject is to provide a D.C. track circuit wherein presence of a series of output pulses provides a positive indication that a track section is unoccupied Vand operative.

The invention contemplates a track circuit comprising a section of rails, a source of current connected to the rails, output means, and saturable means comprising at least one ferrite core composed of substantially square hysteresis loop material coupled between the rails and the output means.

The foregoing and other objects and `advantages of the invention will become apparent from the following detailed description when read Iin conjunction with the accompnying drawings, in which:

FIG. l is a schematic diagram of a typical track circuit embodying the present invention.

FIGS. 2A, 2B, 2C and 2D are diagrams illustrating saturated flux paths in the multi-aperture core under various circuit conditions.

FIG. 3 is a graphical illustration of pulse sequences in the circuit.

FIG. 4 is a schematic diagram of a typical block signalling system incorporating the novel track circuit.

The system of FIG. l is shown comprising a suitable direct current source Stich as a battery, a rectified filtered alternating current source, or the like, applying current to a track section 3 defined by insulators 4, through a limiting resistor 2. This resistor is shown adjustable, to aid in initially producing `a proper value of prime current for core 10. A first clock pulse generator A produces current pulses at an audio frequency rate over a conductor 6 wound through the minor aperture 11 of a first multi-aperture ferrite core 10` and the major aperture 109 of a second multi-aperture ferrite core 110. These pulses comprise set pulses for core 1l) and clear pulses for core 110.

A second clock pulse generator B produces current pulses over a winding 8 threading the major aperture of core lll. These pulses comprise clear pulses :for core 10. Clock generator B may betriggered by clock generator A as shown in FIG. l, in order to provide clock pulses at the same frequency or repetition rate as the clock A pulses, but at a different phase relation, although any circuit for producing constant frequency pulses at different phase relations may be `used in place of individual clock generators A and B.

A prime winding 13 receives direct current from track 3 and threads this current through a second minor aperture 12 of core lil. A loop 14 is wound through minor aperture 12, and a minor aperture 111 of core 110, for coupling pulses from core 10 to core 110.

A radio-frequency signal generator C provides pulses `on a winding 108 threading a minor aperture 112 in core 110. These pulses provide prime and non-destructive read-out pulses for core 110. A second winding 114, which threads minor aperture 112 has induced therein output voltage pulses at a radio-frequency rate. These pulses are rectified through a diode 5 and applied to a relay 115 having contacts 116 and 117. A capacitor 7 is connected in parallel with the relay in order to provide energy storage for maintaining the relay energized during production of output pulses on winding 114. The circuit is connected so as to periodically energize and deenergize relay 115. This will be discussed, infra.

Relay 115 in turn controls energization of a second relay 15. This is achieved in the following manner. When relay 115 is energized, front contacts 116 and 117 of relay 115 provide a charge path for capacitor 120 in series with .a resistor 118. Relay 15, at this time, remains deenergized, since no current is supplied thereto. Then, when relay 115 next deenergizes, back contacts 116 and 117 of relay 115 provide a discharge path for capacitor 120 through relay 15 and resistor 118 in series. Thus, relay becomes energized. When relay 115 next energizes, front contacts 116 and 117 again permit charging of capacitor 120 directly from the voltage source,

A capacitor 119 is connected in parallel with relay 15 to provide energy storage during the interval in which relay 11S is deenergized and capacitor 120 provides energization for relay 15, Thus, when relay 115 again energizes, the voltage source in cooperation with energy stored on capacitor 119 maintains relay 15 in the energized condition. Should capacitor 120 lose its charge, relay 15 will deenergize even though there is a complete circuit from the voltage souce through resistor 118 to relay 15, since current from the positive source, by itself, is insuicient to maintain the relay energized. Therefore, in order for relay 15 to remain energized, relay 115 must continue to move. If relay 115 should remain steadily energized, capacitor 119 loses its charge and relay 15 deenergizes. On the other hand, should relay 115 remain steadily deenergized, capacitor 120 loses its charge through relay 15, resulting in deenergization of relay 1S.

The foregoing components, exclusive of those directly connected to track 3, are combined in what is hereinafter referred to as core `control network 17. Output of the network is provided by front contact 16 of relay 15. It should be noted that only single turns are represented as being wound around the cores shown in FIG. l, for clarity; however, windings of any number of turns may be used, as will be understood by those skilled in the art.

In operation, assume first that track 3 is unoccupied and electrically non-defective. Assume also that clocks A and B are producing pulses at identical frequencies and at a phase relaton as illustrated by FIG. 3. A steady direct current comprising a prime signal is coupled from battery 1 through track 3 to core 10 where it tends to establish a counterclockwise saturated magnetic flux path, such as illustrated by the dotted path in FIG. 2C. This flux direction may be determined by application of the Well-known Right-Hand Rule for determining direction of flux path along a current-carrying conductor. However, when the core is in a clear condition, this flux path around minor aperture 11 cannot be established, since the amplitude of magnetomotive force required to prime the core is below the minimum required to set the core. Thus, the prime current flows in a direction which would tend to increase flux in the core between the major aperture and minor aperture 12. However, no increase of flux is possible in this portion of the core, since it is already saturated. Hence, there can be no flux flow due to prime current in a clear core, since there exists no closed path through which this flux may flow.

When a clock A pulse threads minor aperture 11 in a clear core containing saturated magnetic flux paths as identified by arrows in FIG, 2A, it tends to establish a saturated flux path as illustrated by the dotted arrows and dotted line in FIG. 2B. This path actually is established since the amplitude of set current is greater than that of prime current, and hence a greater length of saturated flux path is produced. In this fashion, flux flow created by the set current takes place through a closed path encompassing the major aperture and minor aperture 11, reversing the direction of flux in the core outside minor aperture 11 and between the major aperture and minor aperture 12. Comparing FIG. 2A with FIG. 2B, it is apparent that the flux path in the outer leg of core 10 outside minor aperture 12 does not change direction when the condition of the core is changed from clear to set. This is because the amplitude of set current is held below a predetermined maximum level in order to avoid such eventuality. Flux flow, therefore, does not take place through this portion of the core, and merely reverses the direction of flux in the Icore between the major aperture and minor aperture 12. Furthermore, since no increase of flux is possible in the portion of the core between the major aperture and minor aperture 11, there is no reversal of the saturated flux direction in this portion of the core. Therefore, when the condition of the core is changed from clear to set, inasmuch as the flux in the outer leg of core 10 outside minor aperture 12 links loop 14, no voltage is produced in the loop because there has been no ux reversal through the loop.

Immediately after occurrence of a core 10 set pulse produced by clock A, the prime current reverses flux direction around minor aperture 12 so that the flux around this aperture now assumes a coun-terclockwise direction, as shown dotted in FIG. 2C. This flux revers-al is made possible by the fact that the set current has previously reversed the direction of set flux in the core outside minor aperture 11 and between the major aperture and minor aperture 12. Although the change in ftux direction due to the prime current links loop 14, the change occurs gradually, d-ue to the relatively slow pulse decay rate of clock A pulses as indicated in FIG. 3, thereby producing substantially no voltage on loop 14. The condition of the core is now that as illustrated in FIG. 2D. Then, when a clock B or core 10 clear pulse is next applied, the core saturates in the clockwise direction, as shown in FIG. 2A, since the amplitude of clear current is considerably greater than the amplitude of set current. Hence, the flux linking loop 14 abruptly changes direction, producing a current pulse in the loop. In this manner, pulses on loop 14 are produced simultaneously with clock B pulses applied to Winding 8, as illustrated in FIG. 3.

The pulses on loop 14 comprise set pulses for core 110. Clear pulses for core are provided by winding 6. When core 110 is set, flux reversal at la radio-frequency rate around minor aperture 1112 of the core is achieved by alternate prime and non-destructive readout pulses produced by R.F. generator C. When core 110 is clear, no flux reversal around minor aperture 112 can occur due to the Vlow amplitude of RF. current produced by generator C, for reasons already described in conjunction with the description of the effect of prime current on a clear core. It should be noted that the maximum amplitude of R.F. current supplied by generator C is substantially equal to that Iof the prime current previously discussed, since flux established in the core due to the prime current -acts in the same direction as that due to the set current. Thus, when core `110 is set, `a voltage is induced in Winding 114 at a radio-frequency rate, due to flux reversal around minor `aperture 112. The waveform of this voltage, which comprises the core 110 output voltage, is illustrated in FIG. 3. This voltage is rectified through diode 5 and applied to relay 115. Thus, relay 115 is periodically energized by bursts of radiofrequency energy `as long as .a prime current is applied through minor aperture 12 of core 10, clock pulses are produced by generators A `and B, and a radio frequency signal is produced by R.F. generator C. Should there be a failure of any of the aforementioned sources of energy, or should any of the conductors in the circuit become open or short-circuited, relay 115 will continuously deenergize. This will cause deenergization of relay 15, as previously explained, opening contact 16 which provides the output of core control network 17 of the track circuit. Thus, the circuit can be seen to be failsaife, in that any .failure in the circuit as well as presence of a train `on track section 3, causes contact 16 to open and thereby change the output of core control network 17.

FIG. 2D shows the magnetic linx directions in core 10 after both a set pulse and prime .signal have been applied to a clear core. Although the set :pulse applied to a clear core tends to Vcreate iiuxes in the direction shown dotted in FIG. 2B, the prime `current flowing through aperture .12 reverses the direction `of ilux path in the core around aperture 1Q as previously explained, causing linx directions as illustrated by arrows in FIG. 2D. Thus, for the condition Where track 3 is unoccupied and operative, saturated ilux paths in core periodically alternate between conditions illustrated in FIGS. 2A, 2B and 2D, in ythat order. By comparing these figures, it can be :seen that the direction of linx in the outer leg of co-re 10 outside aperture y12 alternately reverses direction. This alternating ux linking loop 14 therefore produces unidirectional set pulses for core 110 lat a .frequency identical to that of clocks A and B each time flux conditions in the core change trom those illustrated in FIG. 2A. Substantially no voltage is produced during a change in core llux .conditions from those illustrated in FIG. 2B to those illustrated in FIG. 2C beca-use of the relatively slow clock A or set pulse decay rate, as previously mentioned.

If a defect such as a short or open circuit should occur on track 3, or if a `train should occupy track `3 and thereby shunt battery 1, prime current will not thread minor aperture 12, and core 10 will therefore alternate between the flux conditions shown in FIG. 2A and the ilux conditions shown `in FIG. 2B. In such case, the linx linking loop 14 `does not change direction or magnitude despite application of set and clear pulses to core 10, and hence, loop 14 produces no voltage for setting core 110; therefore, no output voltage appears `on wind-ing 114 and relay 115 is thereby deenergized, in turn deenervgizing relay `15. Contact `16 thus opens.

It should here be noted that the prime lcurrent required to properly operate core 10 must be of sniciently high amplitude to switch the direction of ilux in the core around aperture 12 from a clockwise to counter-clockwise direction. The same is true `for the radio-frequency signal applied to core 110 through aperture `112. Because of t-he square hysteresis loop characteristic `of the ferrite material comprising the cores, a prime current amplitude in excess of this minimum value will merely serve to drive the portion of the core around minor aperture 12 or 112 lfurther into saturation provided it is not sufficiently high to set a clear core. Thus, if the track circuit is designed so as to provide a minimum prime current amplitude suicient to reverse the direction of ilux in :core 10 around minor aperture 12 and a maximum prime current amplitude insufficient to set the core when clear, regardless of changes above a predetermined value of ballast resistance and below a predetermined value of rail resistance, and regardless vof a predetermined amotmt of galvanic action, the track circuit will reliably respond to the presence of a train and be free from erratic operation due to relatively minor changes in rail and ballast resista-nce and relatively minor galvanic action between the rails and ballast.

The voltage produced by output winding 114 may be utilized directly in certain forms of detecting equ-ipment. On the other hand, the voltage on winding 114 may be rectified and directly applied to relay 115, which then produces a pulsating output during the presence of voltage pulses on winding 114, and zero output dur-ing the presence of no pulses on the output winding. The circuit is fail-safe, inasmuch as loss of any of the signals applied to cores 1t) or 110 will cause no output voltage to appear on loop 14, preventing core 110 from becoming set. For example, if a pulse from `loop 14 should set core 110 just prior to Vfailure of loop 14 due to an open or short circuit, core will produce an output only until a clear pulse is produced by clock A. At this time, core 110 will cease producing an output. On the other hand, should winding 6 become open or shorted, core `10 cannot become set, and therefore loop 14 will not set core i110, causing cessation of output from core 110. In addition, failure of clock A -immediately after core 110 is set causes relay 115 to steadily energize. As previously explained, this situation also causes deenergization of relay 15. Furthermore, even if the frequencies of clock A and B pulses should fall out of step with each, output energy of core 110 will decrease, due to shortened intervals of output voltage as well as decreased repetition of these intervals, causing relay to deenergize.

One form of system utilizing the novel track circuit is the block signalling system shown in FIG. 4. This system is a two-block, three-aspect signalling system with neutral line circuit. Each block is divided into three track circuits 26, 27 and 28. Each of these track circuits is connected to a co-re control network 19, 2() and 21 respectively. A fourth control network 18 is shown connected to the rear track circuit of the next block for a westbound train. These core control networks are identical in circuit conguration to core control network 17 of FIG. 1. Contacts 22, 23, 24 and 25 are actuated from core control networks 18, 19, 20 `and 21 respectively. Contacts 23, 24 and 25 are connected in series to control ope-ration of a relay 32 having contacts 33 and 34. Contact 22 is connected to actnate a relay 29 having contact 30 and 31. Contact 30 controls energization of a relay 35 having a contact 36. A pair of signal lights 39 and 410 are shown located along track 3. Front and back contacts 36 control energization ofthe green and yellow signals respectively of signal light 40 through leads not shown. Front contact 34 controls energization of contact 36 while back contact 34 controls energization of the red signal of signal light 40 through leads not shown. Likewise, front and 'back contacts 38 control energization of the green and yellow signals respectively of signal light 39, through leads not shown. Front contact 31 controls energization of con- .tact 38 while back Contact 31 controls energization of the red signal of signal light 39 through a lead not shown.

In operation of the block signalling system, assume a westbound train on track 3 is .approaching signal 40. Presence of a train on any of track circuits 26, 27 or 28 will cause deenergization of the relay in core control network 19, 20 and 21 respectively, as explained previously in conjunction with FIG. 1. The deenergized core control network relay will open its associated contact 23, 24 or 25, thereby deenergizing relay 32. This causes back contact 34 to energize the red signal light of signal 40 and thereby produce an indication of block occupancy for the westbound train. If this block is clear, however, and the next block is occupied, relay 29 will be deenergized, causing back Contact 31 to energize the red signal light of signal 39. At the same time contact 3l) opens, deenergizing relay 35 which causes back contact 36 to complete a circuit to the yellow signal light of signal 40. Relay 32 in this case remains energized so that contact 34 provides energization for the yellow light of signal 40u When both the previously described blocks are unoccupied, relays 29, 32, 37 and 35 are all energized, as shown in FIG. 4, and therefore green signals are indicated at both signal lights 39 and 40. It should be noted that contact 33 of relay 32 serves a function similar to that of contact 30 of relay 29, while relay 37 serves a function similar to that of relay 35.

Thus, there has been shown a fail-safe highly sensitive D.C. track circuit utilizing multi-aperture ferrite cores, which can be used as a building block in any system requiring a positive indication of track condtion. The circuit can be used for driving additional ferrite cores as Well as for energizing a relay. The invention provides a stable,

rugged, long-lasting D.C. track circuit which is relatively unaffected by ballast and rail resistance changes as well as by galvanic action between the ballast and rails.

Although but one specific embodiment of the present invention has been described, it is to be expressly understood that this Vform is selected to facilitate in disclosure of the invention r-ather than to limit the number of forms which it may assume; various modifications and adaptations may be applied to the specific form shown to meet requirements of practice without in lany manner departing from the spirit or scope of the invention.

What I claim is:

1. In combination in a railway track circuit including track Arails and a source of current connected to the ra-ils, a ferrite core having a major aperture and at least two minor apertures, a first current pulse generator of constant frequency magnetically coupled to the clore through the major aperture, a second current pulse generator of constant frequency magnetically coupled to the core through one minor aperture, means magnetically coupling the rails to the core through the other minor aperture, and output means magnetically coupled to the core through said other minor aperture and providing an output signal when the rails are unoccupied.

2. A D.C. track circuit comprising a section of rails, `a source of direct current connected to the section, a ferrite core having `a major aperture and at least two minor apertures, means magnetically coupling current pulses of a given frequency and phase to the core through the major aperture, means magnetically coupling current pulses of said given frequency although shifted in phase from said given phase to the core through one minor laperture, means magnetically coupling direct current from the rails to t-he core through the other minor aperture, and output means magnetically coupled to the core through said other minor aperture so as to produce a series of electrical pulses of said lgiven frequency and phase -on the output winding only when the rails are both unoccupied and continuous.

3. A D.C. track circuit comprising a pair of rails, means applying direct current to the rails, a ferrite core having a major aperture and at least two minor apertures, means magnetizing the core with a series of current pulses coupled through the major aperture and a series of current pulses coupled through one minor aperture, each series of current pulses having the same frequency but different phase as the other, means magnetizing the core with direct current coupled from the rails through the other minor aperture, and a winding coupling said other minor aperture and having induced therein an output signal when the rails are unoccupied.

4. In combination in a railway track circuit including track rails and a source of current connected to the rails, a multi-aperture ferrite core, a first electrical pulse generator of constant frequency magnetically coupled to the core through a first aperture, a second electrical pulse generator operating at the same frequency and magnetically coupled to the core through a second aperture, means magnetically coupling the rails to the core through a third aperture, and output means magnetically coupled to the core through the third aperture and providing output voltage pulses at said frequency when the rails are unoccupied.

5. In combination in a railway track circuit including track rails and a source of current connected to the rails, a multi-aperture core composed of square hysteresis loop material, means coupling electrical pulses of a given repetition rate to the core through a first aperture, means coupling electrical pulses produced at the same repetition rate to the core through a second aperture, means coupling current from the rails to the core through a third aperture, and output means coupled to the core through the third aperture and having induced therein an output signal comprising electrical pulses at said repetition rate only when the rails are both unoccupied and continuous.

6. A D.C. track circuit comprising a pair of rails, means applying direct current to the rails, a multi-aperture core composed of square hysteresis loop material, means magnetizing the core with clear clock current pulses coupled through a first aperture and set clock current pulses coupled through a second aperture, each group of pulses having identical repetition rates but different phases, means connected to the rails and magnetizing the core with said direct current coupled through a third aperture, and means coupling the third aperture and having an output signal induced therein when the rails are unoccupied.

7. A D.C. track circuit having substantially the same value of energization and deenergization voltage comprising a pair of track rails, means applying direct current t0 the rails, a multi-aperture ferrite core, means coupling constant frequency clear pulses to the core through a first aperture, means coupling constant frequency set pulses to the core through a second aperture, means coupling a prime current from the rails to the core through a third aperture, and means coupled to the core through the third aperture and having induced therein a pulsating output signal when the rails are unoccupied.

8. In combination in a railway track circuit including track rails and a source of current connected to the rails, a core of square hysteresis loop material magnetically coupled to the rails, means magnetically coupling a rst series of pulses to the core, means magnetically coupling a second series of pulses to the core, and output means magnetically coupled to the core and providing an output signal only when the rails are unoccupied.

9. In combination, in a railway .track circuit including track rails and a source of current connected to the rails, a pair of ferrite cores, each of said cores having a major aperture and at least two minor apertures, a rst current pulse generator of constant frequency magnetically coupled to a rst core through the major aperture, a second current pulse generator of said constant frequency magnetically coupled to the rst core through one minor aperture and the second core through the major aperture, means coupling the rails to the rst core .through the other minor aperture of the first core, means magnetically coupling the lirst core to the second core through said other minor aperture of the first core and a first minor aperture of the second core, means coupling a signal of frequency higher than said constant frequency to the second core through the Asecond minor aperture, and output means magnetically coupled to the second core through the second minor aperture of the second core and providing an output signal only when the rails are both unoccupied and continuous.

10. A D.C. track circuit comprising a section of rails, a source of direct current connected to the section, a pair of ferrite cores, each of said cores having a major aperture and at least Itwo minor apertures, means magnetically coupling current pulses of a given frequency and phase to the first core through its major aperture, means magnetically coupling current pulses of said given frequency although shifted in phase from said given phase to the first core through one minor aperture and the second core through the major aperture, means magnetically coupling direct current from the rails to the first core through the other minor aperture of said first core, means magnetically coupling the second core to the first core through a rst minor aperture of the second core and said other minor aperture of the first core, means coupling a radio frequency signal through a second minor aperture of the second core, and output means magnetically coupled to the second core through the second minor aperture of the second core so as to produce an output signal only when the rails are both unoccupied and continuous.

11. A D.C. track circuit comprising a pair of rails, means applying direct current to the rails, a pair of ferrite cores, each of the cores having a major aperture and at least two minor apertures, means magnetizing each of the cores with a series of current pulses coupled through the major aperture of each core, means magnetizing the first core with a series of current pulses coupled through a minor aperture, means magnetizing the second core with a series of pulses produced by the rst core, means magnetizing the first core with said direct current, and output means coupled t-o the second core and having induced therein an output signal when the rails are unoccupied.

12. In combination in a railway track circuit including track rails and a source of current connected to the rails, a pair of multi-aperture cores composed of square hysteresis loop material, a first electrical pulse generator of a given repetition rate magnetically coupled to the rst core through `a iirst aperture, a second electrical pulse generator operating at the same repetition rate and magnetically coupled to the first core through a second aperture and the second core through a rst aperture, means magnetically coupling the rails to the rst core through a third aperture, means magnetically coupling the second core to the rst core through the third aperture of the first core and a second aperture of the second core, a signal generator magnetically coupled to the second core through a third aperture, and output means magnetically coupled to the second core through the third aperture and providing an output only when the rails are both unoccupied and continuous.

13. In combination in a railway track circuit including track rails and a source of current connected to the rails, a pair of multi-aperture ferrite cores, means coupling electrical pulses of a constant frequency to the tirst core through a rst aperture, means coupling electrical pulses at the same frequency to the rst core through a second aperture and to the second core through its first aperture, means coupling current produced by said source from the rails to the rst core through a third aperture, means magnetically coupling the first core to the second core through the third aperture of the rst core and a second aperture of the second core, means coupling a signal of frequency higher than said constant frequency to the second core through a third aperture, and means providing output voltage pulses from the second core when the rails are unoccupied, each said output voltage pulse comprising a burst of said higher frequency signal.

14. A D.C. track circuit comprising a section of rails, a source 'of direct current connected to the section, n pair of multi-aperture ferrite cores, each said core having a major aperture, means magnetically coupling one of said ferrite cores to the section of rails, means magnetically coupling the second of said cores to the r'irst of said cores, means coupling current pulses to a constant frequency through the major aperture of the first of said cores, means coupling current pulses of constant frequency through the major aperture of the second of said cores, combined priming and readout means magnetically coupled to the second of said cores, and output means magnetically coupled to the second of said cores.

15. A track circuit comprising a section of rails, a source of direct current connected to the rails, a pair of cores composed `of substantially square hysteresis loop material, one of said cores being magnetically coupled to the rails, the second of said cores being magnetically coupled to the first of said cores, and output means magnetically coupled to the second of said cores and providing an output signal when the rails are unoccupied.

16. The track circuit of claim 15 having additional means magnetically coupling at least one series of electrical pulses to each of said cores.

17. A direct current track circuit comprising a section of track rails, a source of direct current connected across said rails at one end of said section, a core of magnetizable material having substantially a square hysteresis loop characteristic, means connecting a prime winding on said core across the track rails at the other end of said section, output responsive means, means for connecting an output Winding of said core to said out put responsive means, and means including pulse producing means for inductively causing said output winding to activate said output means only when the current from said source flowing through said prime winding is above a predetermined value.

References Cited by the Examiner UNITED STATES PATENTS 2,077,932 4/1937 James 246-41 2,968,795 l/196l. Briggs et al. 340-174 2,983,906 5/1961 Crane 340-174 2,994,069 7/1961 Rajchman etal 340-174 3,0l4,204 l2/l96l Lo et al. 340-174 3,034,108 5/1962 Bennion 340-174 3,076,182 l/l963 Block 340-174 ARTHUR L. LA POINT, Primary Examiner.

LEO QUACKENBUSH, Examiner'.

S. B. GREEN, Assistant Examiner. 

1. IN COMBINATION IN A RAILWAY TRACK CIRCUIT INCLUDING TRACK RAILS AND A SOURCE OF CURRENT CONNECTED TO THE RAILS A FERRITE CORE HAVING A MAJOR APERTURE AND AT LEAST TWO MINOR APERTURES, A FIRST CURRENT PULSE GENERATOR OF CONSTANT FREQUENCY MAGNETICALLY COUPLED TO THE CORE THROUGH THE MAJOR APERTURE, A SECOND CURRENT PULSE GENERATOR OF CONSTANT FREQUENCY MAGNETICALLY COUPLED TO THE CORE THROUGH ONE MINOR APERTURE, MEANS MAGNETICALLY COUPLING THE RAILS TO THE CORE THROUGH THE OUTER MINOR APERTURE AND OUTPUT MEANS MAGNETICALLY COUPLED TO THE CORE THROUGH SAID OTHER MINOR APERTURE AND PROVIDING AN OUTPUT SIGNAL WHEN THE RAILS ARE UNOCCUPIED. 